Laser diodes are the most ubiquitous types of lasers available today. They are compact, rugged and relatively inexpensive. In general, however, laser diodes are characterized by low power outputs that limit their applications. Moreover, they typically operate in only the near-infrared portion of the spectrum. Output powers available from single-mode laser diodes such as single-stripe diodes are as high as two hundred milliwatts (mW), whereas the output powers available from multi-mode laser diodes such as laser diode arrays are on the order of tens of watts.
Because of their relatively low power outputs, it is known to use an external passive optical resonator to enhance the power of single stripe laser diodes. An example of resonant power enhancement of a single-stripe laser diode can be found in U.S. Pat. No. 4,884,276 to Dixon et al., wherein coherent radiation from a laser diode is focused along a beam path and injected into an external confocal Fabry-Perot optical cavity that contains a non-linear optical material, which results in the generation of a harmonic of the cavity radiation that is discharged through the output mirror of the passive cavity.
In a properly-designed passive optical resonator, it is possible to achieve power levels within the resonator that exceed the incident power from the laser diode by several orders of magnitude. This intracavity power multiplication can be used to increase the efficiency of non-linear optical processes (e.g., second harmonic generation, sum frequency generation, difference frequency generation and Raman processes) or resonantly pump a laser medium. Resonantly enhanced sum frequency and second harmonic generation are two of the methods currently being investigated as possible sources of efficient blue and green coherent light. Such lasers are needed for reprographics, video displays, optical data storage, biomedical diagnostics and many other applications. Resonant pumping is a method for improving the efficiency of quasi-three level lasers and pumping weakly-absorbing laser gain media.
The importance of resonant power enhancement of semiconductor-based lasers can be more fully appreciated by considering the second harmonic generation of a semiconductor laser diode in potassium niobate. Commercially available, diffraction-limited diode lasers operating near 860 nm have a maximum output power of approximately 150 mW. A single pass of 150 mW of output power through a five (5) millimeter (mm) crystal of potassium niobate results in output power at the second harmonic that is significantly less than 1 mW. However, if the same crystal is placed in a properly designed optical resonator, the intracavity field will have a power of greater than 10 watts. Because the harmonic efficiency increased linearly with input power, the output of the resonant device of the harmonic frequency can be as high as 70 mW. Thus, using the same laser diode and non-linear crystal, the harmonic output power is increased by 100 to 1000 times when a resonant cavity is employed.
In order to lock the frequency of a laser diode to the resonant frequency of an associated passive resonant cavity, two passive locking techniques employing optical feedback are known. The first of these techniques is commonly called "weak locking" in which a small fraction of the intracavity field of the passive resonant cavity is fed back to the diode laser. This technique is employed in the illustrated embodiment of the above-identified Dixon et al. patent.
In the "weak locking" technique, the ratio of feedback power to the diode laser output power is less than five (5) percent and, in practice, is usually below one (1) percent. This "weak locking" technique narrows the emission spectrum of the laser diode and locks its frequency to the passive cavity over a certain range of ambient conditions (e.g., temperature and vibration) that may effect the phase of the feedback radiation and the length of the optical cavity. However, the range of feedback phase and optical cavity length over which the laser diode and cavity remain locked is relatively narrow with respect to the range that can be expected in a typical commercial operating environment. For example, exposure to environmentally normal thermal variations and vibrations may result in the diode laser being unable to remain at all times locked to the cavity. As a result, an additional electronic feedback loop is required to keep the amplitude of the intracavity field constant in all but the most carefully controlled environments.
A second locking technique called "strong locking" is an improvement with respect to the "weak locking" technique in that it can lock the diode laser to the passive cavity regardless of changes in the phase of the feedback radiation and the length of the cavity. Thus, this "strong locking" technique overcomes the frequency instability problems experienced by the "weak locking" technique for devices operating in uncontrolled environments. In order to employ the "strong locking" technique, the output facet of the laser diode is usually anti-reflection coated (e.g., a reflectivity of below 10.sup.-3) and an optical feedback signal with an amplitude greater than five (5) percent of the diode laser output is imaged back into the diode facet. In effect, the addition of the anti-reflective coating to the output facet of the laser diode extends the resonant cavity formed within the semiconductor element of the diode. The external passive cavity is the equivalent output mirror for the extended resonator.
In the "strong locking" technique the passive cavity resonator functions as a transmission filter in the extended linear cavity. The light is double-passed through the passive resonator in order to feed back a portion of the light to the diode. This "strong locking" technique is disclosed in U.S. Pat. No. 5,038,352 to Lenth et al., wherein the light transmitted by the output mirror of the external cavity is reflected by a highly reflecting mirror back through the passive cavity to the laser diode. Even in this "strong locking" technique, however, an electronic feedback loop is required to control the wavelength of the laser diode.
More recently, this "strong locking" technique has been modified by replacing the electronic feedback loop with a frequency-selective diffraction grating. W. J. Kozlovsky, W. P. Risk and W. P. Lenth, "Resonator-Enhanced Frequency Doubling In An Extended Cavity Diode Laser," Digest of the Compact Blue Green Laser Topical Meeting, Addendum and Postdeadline Papers (Optical Society of America, Washington, 1993), Paper PD2. In this work, the optical feedback level was approximately three (3) percent. Although the "strong locking" technique employing a diffraction grating in substitution for an electronic feedback loop is highly effective in locking the frequency of the laser diode to a passive cavity for all possible values of feedback phase and cavity length, applicant has found that the amplitude of the harmonic output is characterized by a signal-to-noise ratio of approximately two to one. For most applications of resonant cavities pumped by laser diodes, an amplitude stability of less than one (1) percent is required.
Although applicant is unaware of any published experiments based on the "strong locking" techniques using an electronic feedback loop as described in the above-identified Lenth et al. patent, applicant's own experiments indicate that an electronic feedback loop cannot eliminate the frequency hopping that is characteristic of "strong locking" in the absence of a frequency selective element in the feedback path. With the use of such a frequency selective element, however, the "strong locking" technique is unable to produce an amplitude-stable field inside a high-finesse external resonator of the type required for the efficient generation of resonant second harmonics, resonant optical mixing or resonant pumping. For example, in second harmonic generators, instabilities in the field within the passive cavity are magnified in the harmonic output since the output is proportional to the square of the intracavity power. In order to produce the amplitude-stable, intracavity field required for practical device applications, improvements to the techniques employed for "strong locking" are needed.
In addition to double-passing the feedback radiation through the passive external cavity as taught in both the Lenth et al. patent and the Kozlovsky et al. publication, the Lenth et al. patent also describes a feedback scheme based on an L-shaped cavity for the external resonator. This configuration is identical to that of a Fox-Smith resonator used to force single mode oscillation of Argon ion and other lasers that normally operate in multiple longitudinal modes. Because the feedback signal is taken directly from the optical cavity, the feedback levels that can be achieved using this design are potentially much higher than those that can be obtained from the double-pass feedback technique.
Unfortunately, it is impossible to impedance match a Fox-Smith resonator and the coupling losses increase rapidly with the finesse of the cavity. In addition, the electronic wavelength control taught by the Lenth et al. patent is ineffective and a frequency selective element must be inserted between the diode and resonator in order to keep the diode emission within the spectral phase-matching bandwidth of the intracavity harmonic process. The insertion of a grating between the diode laser and the external cavity as taught by the Kozlovsky et al. publication can be expected to significantly reduce the amplitude of the feedback signal. The diffraction efficiency of a typical grating is between 60-80 percent. Thus placement of a grating between the diode and Fox-Smith resonator would reduce the fundamental field inside the resonator by 20 to 40 percent and the harmonic output of a resonant doubler by a factor of 40 to 64 percent. Because the feedback signal must be reflected twice by the grating in a Fox-Smith configuration, the amplitude of the optical feedback to the output facet of the diode would be reduced by a similar factor. As with the double-pass feedback scheme, it is difficult to obtain a feedback signal from a Fox-Smith configuration that is sufficiently strong to ensure stable operation.